It is desirable to be able to continuously determine the position of a marine vessel relative to one or more other objects such as another marine vessel or a fixed platform, and a dynamic positioning system can be used for this purpose.
A dynamic positioning system includes a position reference sensor mounted on the marine vessel and FIGS. 1 and 2 are diagrammatic illustrations of the operation of a position reference sensor 10 in the horizontal and vertical planes respectively. The position reference sensor 10 comprises an optical assembly 12 enclosed in a housing mounted on a marine vessel 14. The optical assembly 12 includes a pulsed laser device 16 and associated optics which produce a vertically fanned beam 18 of laser light. The housing and enclosed optical assembly 12 rotate continuously about a generally vertical axis 20, as shown diagrammatically by the rotation line 24, or alternatively oscillate about a generally vertical axis, to sequentially and repeatedly direct the vertically fanned beam 18 of laser light towards retro-reflective targets 22 spaced apart horizontally and fixed to one or more objects. The optical assembly 12 also includes a photodetector 26 and associated optics, located adjacent to the pulsed laser device 16 and its associated optics. The photodetector 26 and associated optics rotate with the pulsed laser device 16 and the photodetector 26 detects laser light 28 reflected by the one or more retro-reflective targets 22.
In operation, the dynamic positioning system determines the range of each retro-reflective target 22 from the optical assembly 12, and hence from the marine vessel 14, based on the time of flight of the emitted and reflected laser light. The dynamic positioning system may also determine the bearing relative to each retro-reflective target 22 based on the rotational position of the optical assembly 12 when the reflected laser light 28 is detected by the photodetector 26. The range and bearing data are both used to determine the position of the marine vessel 14 relative to the retro-reflective targets 22 and, hence, the one or more objects.
A marine vessel 14 is subject to pitch, roll and heave motion, as induced by the prevailing sea state. Because the beam 18 of laser light is fanned (i.e. divergent) in the vertical direction, it is likely that the vertically fanned beam 18 will strike the one or more retro-reflective targets 22 if pitch, roll or heave occur. The beam divergence is, however, limited typically to about ±8° above and below a beam centre line CL (FIG. 2). A gyroscope-based motion sensor and motorised tilt mechanism are thus employed to alter the vertical inclination of the optical assembly 12, and hence the vertically fanned beam 18 of laser light, to compensate further for pitch and roll motion of the marine vessel 14. The motorised tilt mechanism is typically capable of tilting by ±20° above and below the horizontal plane.
Even with the position reference sensor 10 described above, it is possible that one or more of the retro-reflective targets 22 could move out of the field of illumination of the vertically fanned beam 18 of laser light if there is significant pitch, roll or heave motion of the marine vessel 14. It is thus possible that the position reference sensor may be rendered inoperable, even if only temporarily. There is, therefore, a need for an improved position reference sensor for a marine vessel which compensates for pitch, roll and heave motion of the marine vessel.